Semi-submersible heavy lift ships, also known as float-on/float-off ships, are known for transporting very large and/or heavy cargo such as drilling rigs or other vessels etc. These ships are submerged in water such that the water line is above the deck level of the ship, the cargo is loaded and the ship lifted to its normal transit position. The propellers of these ships thus may need to be retracted within the baseline of the ship, enabling the vessel to enter shallow waters (harbours), as well as reducing resistance when not in use during transit.
Retractable thruster arrangements are known in the art and described for example in the documents U.S. Pat. No. 6,439,936 and U.S. Pat. No. 6,375,524. In the document U.S. Pat. No. 6,439,936, the thruster is however elevated above the ship's deck level. This reduces the usable surface area of the deck and by changing the ship's centre of gravity, also makes it more unstable. The document U.S. Pat. No. 6,375,524 describes another retractable thruster that is elevated above the ship's deck level for servicing of the propulsion device. The document describes an intermediate position wherein the thruster is retracted to keel level without any significant structure projecting along the deck level. However, in this intermediate position, there is no possibility to seal the device such that the propeller could be serviced.
The document WO 2010/100313 also describes a retractable thruster comprising a closing device that allows the servicing of the propulsion unit even when the propulsion device is below the water line of the vessel. This kind of equipment is however quite expensive and therefore not economical in all kinds of vessels. Morover, document U.S. Pat. No. 2,466,635 discloses a screw propeller steering and depth regulation apparatus, wherein the propeller can be withdrawn within the contour of the hull when the vessel is in shallow water or is to be propelled by other means. The apparatus has a well-like structure which carries the propulsive unit telescoped in a three-part telescopic well structure. The well structure has two sealing rings in annular grooves in the exterior of the well sections. Document WO 2011/031158 discloses a vessel with a retractable thruster assembly, wherein its casing structure comprises also two sealing devices arranged on the sides of the thruster well.